The invention relates to an apparatus for drying galvanized mass-produced parts disposed in an immersion-type drum which drum is rotatable around its horizontal longitudinal axis and has a perforated shell. More particularly, the invention is in an apparatus comprised of a container having a cavity for accommodating the drum, and a chamber for guiding a stream of drying-air which passes through the charge of mass-produced parts and is produced by suction or blowing action of a blower, wherein the chamber is divided into two separated regions one of which is upstream of the drum and is intended to guide the stream of dry air, and the other of which is downstream of the drum and is intended to guide the stream of humid air.
Such drying apparatuses are known, from German Patents Nos. 2,803,117; 2,836,183; and 3,029,520. They serve to dry charges of pourable mass-produced parts in an immersion drum following final electrochemical or chemical surface treatment and final rinsing. In such drying in the same drum previously used for the surface treatment, a number of process-specific features which occur simultaneously must be taken into account, particularly the following two processes which at least partially overlap:
1. The drawing along and entrainment of the liquid drops of various sizes adhering to the surfaces of the mass-produced parts by the stream of drying-air incident on the surfaces as air passes through the immersion-type drum and through the charge; and
2. The evaporation of the residual liquid film on the surface of the mass-produced parts under the action of the stream of drying-air which has been heated to about 60.degree. C. and is passing over the surfaces of the mass-produced parts.
The first above phenomenon should be given highest priority in the drying. If the predominant phenomenon in the drying is the second listed above, undesirable spotting will occur on the surfaces of the mass-produced parts.
The spotting is comprised primarily of residues of mineral salts from the rinse water which salts are dissolved in the liquid film and which solidify as a consequence of the drying. To minimize such spotting, the speed of the entrainment of the liquid droplets adhering to the surfaces of the mass-produced parts must be maximized, and at the same time the evaporation must be minimized. This necessitates that the stream of drying-air pass through the charge at maximum speed. In order to be able to generate such a stream, sources of resistance in the flow path should be avoided.
As a rule, the stream of drying-air which passes through the perforated shell of the drum and through the charge is passed in a circulating loop. A precondition of this is that the liquid droplets which are entrained from the surfaces of the mass-produced parts be removed from the air stream and from the flow path of the stream, so that the droplets cannot be returned to the charge which is undergoing drying. In German Patent No. 2,803,117 there is no discussion of the removal of moisture from the stream of drying-air. In the dryer according to German Patent No. 2,836,183 moisture is removed by a drop separator which the humid air stream reaches through a lengthy path. Because no appreciable moisture removal occurs on the path, the moisture removal must occur practically exclusively in the drop separator. Consequently, the separator must be large and must impose a substantial pressure drop on the stream of drying-air.
In the dryer according to German patent No. 3,029,520 a draining pan is disposed below the immersion-type drum. The intention is that liquid droplets entrained in the stream of drying-air will be removed with the aid of this pan. However, the removal is imperfect. The humid air stream which leaves the drum in an essentially vertical downward direction passes across the middle of the draining pan, and therefore most of the drops contained in the stream are passed to the blower where they are swirled into a fine mist and are passed backed to the charge. Also the draining pan imposes substantial flow resistance which results in appreciable reduction of the flow speed and prolongation of the time to accomplish the drying. As mentioned above, low flow speed and long drying times both promote evaporation of the liquid adhering to the surfaces of the mass-produced parts, thereby promoting spotting. Also, long drying times lead to appreciable mechanical stressing in the drum which is rotating during the drying, and thereby lead to damage to readily damaged surface coatings, e.g. the surfaces of galvanized mass-produced parts, which surfaces often have been passivated.